1. Field of the Invention
The present invention relates to fluid flow control, and, more particularly, to an improved method and apparatus to generate extremely low flow rates at high pressures that are accurately regulated for use in liquid chromatography and mass spectrometry.
2. Description of Related Art
In liquid chromatography, the liquid (solvent) flow rates generally used are between 100 μliters/min and 10 ml/min. However, more widespread use of capillary and micro-bore columns, as well as new applications such as combining liquid chromatography with mass spectrometry has created a requirement for flow rates that are as much as 100 times lower than the above mentioned flow rates. Very few HPLC pumps are capable of delivering accurate, stable flow rates in this range, and those that can do so are extremely expensive. As a result, most manufacturers have adopted a method known as flow splitting to achieve low flow rates with conventional high pressure liquid chromatography pumps. In the prior art, flow splitting is accomplished by using a tee to split the flow into two paths. Capillary tubes of different lengths and/or diameters are used on each path to create different fluid resistance in each path. Because these flow rates are all in the laminar flow regime, the resulting flow rates in each path can be found from the following relation:
      Δ    ⁢                  ⁢    P    =            μ      ⁢                          ⁢      LQ              d      4      where P is pressure, μ is dynamic viscosity, L is capillary flow length, Q is flow rate, and d is effective capillary diameter.
The significant disadvantages of prior art include:                Capillary tube is easy to clog.        Split ratios may change during a chromatographic run because of changes in fluid viscosity that occur during the run. This may occur because the volume in the capillary tubing can be large enough that the composition of solvents can become different in the two legs when the pre-split composition is changing.        Difficult to adjust split flow ratio. It is necessary to cut the tubing to different lengths to adjust the split ratio. This is difficult to do because the capillary usually closes off due to crimping in the cutting process available to most chromatographers in the lab, and thus the fluid resistance changes unpredictably. Also, it is necessary to cut and join additional pieces of tubing to add resistance. This is a troublesome and time consuming process because of the small tube and fitting size.        Verifying the flow rate is extremely difficult. There are no commercially available flow meters that work at these low flow rates. Measuring flow rates as low as 100 nanoliters per minute must be done by weighing the effluent on the low path side over a known time interval. This is very time intensive. Using traditional methods, such as pressure drop through a known capillary, do not work because the viscosity of the fluids is often changing over time, and is unknown.When the capillary analytical column is added to the low flow path, the split ratio is changed. The resistance of analytical columns is usually unknown, and will change with a change in solvent viscosity. Thus, it is necessary for the user to measure flow rates after adding the column to the flow splitter.        